Line power converters input alternating current (AC) power and convert it into direct current (DC) power for supplying a load, e.g., charging batteries in smart phones, laptops, or other portable devices, or for otherwise powering an electronic device. Such power converters typically include an input stage, which converts an input AC voltage into an intermediate voltage that is DC, and a switching DC/DC converter, which converts the intermediate DC voltage into an output DC voltage level that is appropriate for powering the load.
Power converters limited to low or moderate power requirements, e.g., below an input power of 75 W, typically require no power factor correction. The input stage for such power converters typically includes a protection circuit, an electromagnetic interference (EMI) filter, a bridge rectifier, and a bulk capacitor. The EMI filter and the protection circuit are connected to an input power source, such as an AC line voltage provided by an electrical mains. The output of the protection circuit and the EMI filter is provided to a rectifier, such as a diode-based bridge rectifier, that outputs a rectified voltage. The rectified voltage is coupled to the bulk capacitor, which stores energy and smoothes the rectified (DC) voltage. More particularly, this smoothed voltage may be characterized as having an average (DC) value with a ripple superimposed on it, wherein the ripple frequency is related to a frequency of the AC power source (e.g., 50 Hz, 60 Hz) and the ripple magnitude is largely determined by the value of the bulk capacitor.
The intermediate DC voltage is input to the switching DC/DC voltage converter. The DC/DC voltage converter includes a transformer comprised of primary and secondary windings. One or more power switches switchably couple the intermediate DC voltage onto the primary winding(s) of the DC/DC voltage converter, thereby producing an AC voltage across the primary winding(s). This induces an AC voltage on the secondary winding(s) of the DC/DC voltage converter. The secondary-side voltage and current are rectified and filtered before being provided to the power converter load. The DC/DC voltage converter operates optimally when its input intermediate DC voltage is constant. In practice, the bulk capacitance is chosen so as to constrain the ripple of the intermediate DC voltage to be within an acceptably narrow range. The DC/DC voltage converter may specify an acceptable voltage range for this input, or may specify a minimum allowed input voltage.
Power converters configured to work with various AC mains voltage levels, i.e., universal input converters, require a bulk capacitor capable of handling fairly large voltages. For example, a capacitor voltage rating of 400V may be necessary to handle line voltages of 340V peak (240 VRMS), as used in much of the world, together with some margin for lightning surges, etc. The bulk capacitor must also have a fairly large capacitance so as to meet the input voltage requirements (voltage ripple and/or minimum voltage) of the DC/DC voltage converter. For example, a capacitance of 1.5 to 2.0 μF per Watt of power rating for the power converter may be required.
The physical size (volume and printed circuit board footprint) of the bulk capacitor within a typical power converter is quite large due to its required capacitance and maximum voltage ratings. The size requirements for DC/DC voltage converters are on a gradual downward trajectory, as their constituent components, e.g., transformer, capacitors, inductors, may be reduced in size as the switching frequency of DC/DC voltage converters increases. However, such scaling does not apply to the bulk capacitor, as the input voltage frequency is fixed to that provided by the power source, e.g., the mains voltage. The bulk capacitor already consumes a large portion of the overall volume for a power converter, and this portion is increasing due to the gradually decreasing size requirements for the DC/DC voltage converter.
One technique for reducing the capacitance of the bulk capacitor is to include a non-isolated boost converter that pre-regulates the input voltage to a much higher voltage (e.g., 400V) that is fed to the DC/DC voltage converter. This is the approach taken by power factor correction (PFC) converters. Because a much higher voltage is provided to the DC/DC voltage converter, a smaller bulk capacitor may be used while still meeting minimum voltage input requirements of the DC/DC voltage converter. However, such an approach has drawbacks in terms of efficiency, as the added conversion stage has associated power loss. Furthermore, the boost converter employs a fairly large input inductor, which largely negates any size reduction of the bulk capacitor. Hence, the complexity and size of an added input stage, e.g., a boost or other PFC converter, makes such approaches undesirable, at least for lower-power converters that do not require PFC.
Circuits and associated techniques are desired that would allow for a reduction in the bulk capacitor used within power converters.